Research On The Control Strategies Of Three-level Buck Converter

With the development of energy technology, power electronic converters havebeen widely used in high power and high voltage output application. Because theswitches suffer only half of the input voltage, the three-level DC-DC converters havebeen attracting more and more attentions. Compared with the ISOS DC-DC converters,the three-level converters have smaller inductor current ripple and use fewer inductorsand capacitors, decrease the number of passive elements obviously, which isconvenient for integration. But in order to achieve the advantage of three-level DC-DCconverter, the flying capacitor voltage should be kept on a stable value. This papermade a study on the three-level Buck (Buck TL) DC-DC converter and its controlstrategies.Accordingly, this paper made a detailed study on working principle andperformance characteristics of Buck TL converter with interleaving control technologyand the factors influenced the flying capacitor voltage. Pointed out that it can notguarantee the stability of the flying capacitor voltage only depended upon the outputvoltage closed-loop control. Aimed at the problem of the stability of the flyingcapacitor voltage, studied two kinds control strategies of the flying capacitor, that is,voltage decoupling control strategy and valley current control strategy.The results show that: both of voltage decoupling control and valley currentcontrol can guarantee to achieve stability of flying capacitor voltage. Voltagedecoupling control by means of the output voltage closed-loop determined the dutycycle, the flying capacitor voltage closed-loop adjusted the duty cycle tidily to keep thestable of flying capacitor voltage. Because inductor current contains the information offlying capacitor voltage, the voltage of flying capacitor can be stabilized by adjustingthe inductor current. Therefore, the flying capacitor voltage can be stabilized bycontrolling the inductor current based on leading-edge valley current control.According to two control strategies, equivalent controlled source circuit methodand state-space averaging method have been used to establish the small signal modelof Buck TL DC-DC converter respectively. Based on the models, compensationnetwork can be designed and calculated to construct the closed-loop systems. Thecurrent control mode was compared with the voltage one, of which the advantage wasverified at low duty cycle. Finally, based on the theoretical research and simulation validation, experimentalverification has been made. Taken the step-by-step strategy, the work waveforms ofconverter were obtained during experimental process. Through analysis of thewaveforms which validated the correctness of theoretical analysis and the effectivenessof control strategies.